Replace camera overlays with dashcam view bar

[danielchalmers]

Summary

• Reverts the previous non-functional PR implementation in a normal revert commit, then replaces it with a fresh camera layout.
• Removes inset camera overlays and deletes OverlayCameraLoader; all available camera MP4s now open directly through Flyleaf and share play/pause/seek/speed operations.
• Adds an in-car-style horizontal view bar under the main video with Grid, Front, Rear, Left, and Right tiles.
• Moves the single Flyleaf host for each camera between the main view, grid view, and thumbnail slots instead of duplicating streams.
• Tightens window shutdown so active Flyleaf streams are stopped/disposed before the window closes.
• Updates playback tests and removes stale FFmpeg overlay/scale filter expectations.

Closes #25.